This invention relates to the field of regulated electrical power supplies and associated control circuits, especially for television apparatus, and more particularly to a regulated electrical power supply wherein periodic and transitory changes affecting loading are sensed, adaptively correlated to their anticipated effect on loading, and used to vary the present output of the power supply to compensate before a variation appears on the output of the power supply, resulting in improved voltage regulation.
Electrical power supplies include, for example, means for converting an input voltage, which may be alternating current in a rectifying power supply, or direct current in a DC-DC supply or inverter, to an output voltage. Typically, a nominally constant DC voltage is desired, notwithstanding current variations. Whereas the power supply has an internal resistance and the current drawn from the supply by one or more electrical loads normally varies with periodic and/or transitory occurrences, the power supply is regulated such that the output voltage remains constant even though the current passing into the load varies. Such regulation is discussed herein with reference to a constant voltage, variable current power supply; however, the same considerations are applicable to constant current, variable voltage supplies as well.
In a typical voltage regulated power supply the present output voltage is sensed and compared to a reference defining a desired output voltage. For example, a differential amplifier has inputs coupled to the output voltage and to a constant voltage reference such as a reverse biased Zener diode. The output of the differential amplifier varies as a function of the difference between the present output level and the desired reference output level, and is known as the error signal or correction signal. The error signal represents the regulation error of the power supply and is used to control the delivery of energy from the input of the power supply to the output, that is, to the loads. In this manner, the power supply seeks to maintain or regulate the output for constant voltage regardless of variations in current with changes in loading. The same sort of technique is applied in a constant current power supplies to maintain a particular regulated current level regardless of changes in loading.
An element of the regulated power supply is responsive to the error signal from the differential amplifier to control the voltage and/or current applied to the output stages of the power supply. In a conventional switched mode power supply often used in television sets, as shown for example in FIG. 3, the output error signal V.sub.e from differential amplifier 201 is coupled to a pulse width modulator 202 to control the amount of energy applied to the primary side of transformer 203. Amplifier 201 typically has an input impedance Z.sub.i and a feedback loop impedance Z.sub.f. Transistor 204 switches current on the primary side, at input voltage V.sub.in, from input voltage source 205. The output at the secondary side of the transformer 203 can be rectified and filtered using diode 206 and capacitor 207 or the like, to produce regulated output voltage V.sub.out. The regulated output is applied to the various loads coupled to the supply, indicated generally as load 210. This regulated output voltage is also coupled to the input of the differential amplifier 201, forming a feedback loop.
An inherent problem with a regulated power supply of the type shown in FIG. 3 is that in the event of a change in loading, a change in the output voltage V.sub.out must actually occur before any change in the application of energy to the primary side of transformer 203 can occur. Because the error voltage is generated by sensing the present level of output V.sub.out, when either the input voltage V.sub.in changes, or the current drawn by the load 210 changes, the output voltage must vary from the nominal output defined by V.sub.ref before a correction can be made in the amount of energy coupled from the input of the power supply to the output.
The magnitude of the variation in the level of output V.sub.out from nominal which occurs in a feedback regulated supply as shown in FIG. 3 depends on the overall closed loop gain of the feedback loop at the frequency at which the load varies or the input voltage varies. The loop gain cannot be made arbitrarily high without compromising the stability of the feedback loop. Moreover, the input voltage and the impedance of the various elements which make up load 210 typically vary periodically at different frequencies and also are subject to transitory variations. Therefore, it is inherent in this form of regulated supply that some change must occur in the output voltage V.sub.out as a function of changes in loading and/or input supply voltage.
In many instances the change in the output loading can be correlated with a change in another independent signal in the system. However, the relationship between the independent signal and the change in load is rarely linear and may change due to interactions of the loads. In a television set, for example, there is a correlation between the incoming video signal and the load presented on the main power supply. Similarly, the load on an audio amplifier correlates in part to the incoming audio signal. It is known to reduce the magnitude of variation in the output of a regulated supply by coupling such a correlated independent signal to the differential amplifier. The idea is to feed forward the independent signal to achieve a needed adjustment in the output of the power supply without waiting for the need for the change to appear as a variation in the level of the regulated output. Examples of such circuits are described in U.S. Pat. Nos. 4,536,700 and 4,809,150.
The limitations of this kind of feed forward approach to power supply regulation as described can be appreciated from the fact that the variation in output level does not exactly and linearly coincide with the variation of the independent signal. Feed forward can only be fully effective if these variables correspond exactly. This form of feed forward regulation is not fully effective because the exact relationship of the output level to other changing variables is neither constant nor readily predicted.
A neural network in a control system can build up over time a series of weight factors representing response of the regulated power supply to the respective load affecting variable(s). The incoming signal for the variable is then sampled and applied to the weight factors defining the response, to provide a calculated feed forward error signal which is used to modify the amount of energy coupled from the power input to the power supply to its regulated output.
A method which can be used to vary the filter weights to optimize regulation is referred to as back propagation and is known in the arts of neural networks and finite impulse response filters. Back propagation refers to a learning rule, not a specific circuit architecture. Generally, however, a back propagation neural network is hierarchical, comprising at least three layers of neurons, also referred to as neurodes. A neuron is a single processing element. In the simplest case, there is an input layer, an output layer and a middle layer. The middle layer is sometimes referred to as the hidden layer, although it is not actually hidden. The input layer must have a buffer element for each of the input signals and the output layer must have a buffer element for each of the output signals. The size of the middle layer can be a matter of design choice and optimization between learning ability and speed of operation. The neurons and layers can be fully connected or selectively connected. When fully connected, every neuron of the input layer is connected to every neuron of the middle layer. Similarly, every neuron of the middle layer is connected to every neuron of the output layer. The neurons of the same layer need not be connected to one another. In operation, a given input results in a certain output, determined by the interconnection pattern of the neurons and the learning rule of the network. The output results in a certain error. The error is propagated back to the middle layer, where the error of each middle layer neuron is computed. The learning rule is then applied to adjust the weight factors before processing the next input. One learning rule, for example the Delta learning rule, is based upon the least means squares, which is a gradient descent learning rule. The term back propagation is sometimes limited to the context of neural networks having more than one middle processing layer, whereas the application of the Delta rule is limited to neural networks having single middle layers. This distinction is of no practical consequence in the context of the inventive arrangements taught herein. Application of a learning rule herein, such as the Delta rule, will be deemed to be an instance of back propagation, insofar as an error signal will be used to adjust the weights of a middle layer in a neural network. Reference may be made to the Neural Network Primer generally, and Part III in particular, available from Al EXPERT, Miller Freeman Publications, San Francisco, Calif.